"The scientific name is Velella velella," said David Bader, director of education at the Aquarium of the Pacific in Long Beach, California. "They're jelly-like creatures, but they're not exactly jellyfish."

Also known as "by-the-wind sailors," the creatures have a clear, distinctive fin sticking up from a blue, oval body no larger than the palm of a hand.

"Every now and then, the currents and the winds will change and these guys will, instead of being pushed out to sea, they actually wind up on the beach," Bader told CNN.

They first started showing up in Washington and Oregon, and by mid-July reached San Francisco. On Thursday, photos turned up on Twitter from Hollywood and Silver Strand beaches in Oxnard.

By-the-wind sailors do sting to catch their food, but humans need not worry.

"That sting is not very potent. It's nothing that could actually get through my skin," Bader said while holding an assortment of the creatures.

Velella velella feeds on plankton and fish eggs caught with tentacles that hang down from its jelly-like body. The creatures float on top of the ocean with their fins sticking straight up, leaving them at the mercy of the wind and the current.

"Out at sea, they look like bubbles on the surface of the ocean until you get up on them," said Julie Bursek of NOAA's Channel Islands National Marine Sanctuary.

They don't live long on the shore, causing their deep blue color to fade until their clear, tougher fins are all that's left behind.

"A lot of people probably never knew an organism like this existed in the world," Bader said. "And you know the winds change, and all of a sudden they wash up on shore and we get to see what the ocean is really made of."